Methods for controlling scale formation in acqueous systems

ABSTRACT

A method of controlling scale formation and deposition and corrosion in aqueous systems is described wherein a treatment solution of a substoichiometric amount of a scale/corrosion inhibitor in combination with a substituted alkylpolycarboxylate is employed. The scale/corrosion inhibitor can be a phosphonate such as hydroxyethylidene diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, hydroxyphosphonoacetic acids, amino phosphonates; terpolymers, copolymers and homopolymers of acrylic acid, maleic acid, epoxysuccinic acids such as polyepoxysuccinic acid or mixtures thereof. The substituted alkylpolycarboxylates are compounds of the general structure: R--X--Y n  wherein R is a hydrocarbon functionality having 4 to 18 carbons; X, when present, is --NH--, --NY 1  --, --N--&gt;O, --N--OC--Y 1 , --O-- or --S--; n is greater than or equal to 1; and Y and Y 1  each independently represent a substituted or non-substituted carboxylic acid functionality such that said alkylpolycarboxylate has at least 2 carboxylic acid functional groups.

This application is a continuation-in-part of application Ser. No.209,806, filed Mar. 11, 1994, now U.S. Pat. No. 5,378,390 which is adivisional of application Ser. No. 099,674, filed Jul. 29, 1993 now U.S.Pat. No. 5,326,478.

FIELD OF THE INVENTION

The present invention relates to the treatment of water to inhibit theformation of scale and corrosion. More particularly, the presentinvention relates to a combination treatment for aqueous systems whichcomprises a scale and/or corrosion inhibitor in combination with asubstituted alkylpolycarboxylate.

BACKGROUND OF THE INVENTION

Although the present invention has general applicability to any givensystem where corrosion and/or the formation and deposition of scale is apotential problem, the invention will be discussed in detail as itconcerns circulating cooling water systems. The present inventionrelates to methods for enhancing the inhibiting properties of scale andcorrosion inhibitors in aqueous systems.

In industrial cooling systems, water such as from rivers, lakes, ponds,etc., is employed as the cooling media for heat exchangers. Such naturalwaters contain large amounts of suspended material such as silt, clayand organic wastes. The cooling water circulating through a heatexchanger is typically circulated through a cooling tower, spray pond orevaporative systems prior to discharge or reuse. In such systems,cooling is achieved by evaporating a portion of the water passingthrough the system. Because of the evaporation which takes place duringcooling, suspended materials in the water become concentrated. Foulingmaterials from the feedwater or as a result of the evaporativeconcentration can settle in locations of low flow rates and causecorrosion and inefficient heat transfer. Agglomerating agents such aspolyacrylamides and polyacrylates have been used to agglomerate fineparticles of mud and silt into a loose floc for removal. However, theseflocs tend to settle in cooling tower basins and frequent cleaning isnecessary to remove the settled flocs from the tower basins.

The water employed in industrial cooling water systems also oftencontains dissolved salts such as calcium, magnesium, etc., which canlead to scale and sludge deposits. One of the most common scale depositsin circulating aqueous cooling systems is calcium carbonate. It normallyresults from the breakdown of calcium bicarbonate, a naturally occurringsoluble salt. Calcium carbonate has a relatively low solubility and itssolubility decreases with increasing temperature and pH. Thus, the rateof calcium carbonate deposition increases with increasing pH andtemperature.

Deposit control agents such as phosphates, phosphonates, polymaleicacids and polyacrylates are often used to inhibit calcium carbonatescale formation in industrial cooling water systems. The use ofpolyacrylates or polymaleic acids alone is not effective at high calciumconcentrations because undesirable polyacrylate- or polymaleate-calciumadducts are formed reducing efficacy. Although phosphonates are veryefficient at controlling calcium carbonate scale formation, they canproduce insoluble phosphonate-calcium complexes or calcium phosphatescale upon degradation.

Preventing the corrosion and scaling of industrial heat transferequipment is essential to the efficient and economical operation of acooling water system. Excessive corrosion of metallic surfaces can causethe premature failure of process equipment requiring down time for thereplacement or repair of the equipment. Additionally, the buildup ofcorrosion products on heat transfer surfaces impedes water flow andreduces heat transfer efficiency thereby limiting production orrequiring down time for cleaning. Reductions in efficiency will alsoresult from scaling deposits which retard heat transfer and hinder waterflow.

Scale can also cause rapid localized corrosion and subsequentpenetration of metallic surfaces through the formation of differentialoxygen concentration cells. The localized corrosion resulting:fromdifferential oxygen cells originating from deposits is commonly referredto as "under deposit corrosion".

The treatment of industrial waters to inhibit scale formation withpolyepoxysuccinic acid (hereinafter PESA) is disclosed in U.S. Pat. No.5,062,962 incorporated herein by reference. The general formula for PESAis: ##STR1## wherein n ranges from about 2 to 50, preferably 2 to 25; Mis hydrogen or a water soluble cation such as Na⁺, NH₄ ⁺ or K⁺ and R ishydrogen, C1-C4 alkyl or C1-C4 substituted alkyl (preferably R ishydrogen). PESA is known to be an effective inhibitor for scale control.

Other materials that are employed to inhibit scale formation and/orcorrosion include phosphate type inhibitors such as hydroxyethylidenediphosphonic acid (HEDP), 2-phosphonobutane-1,2,4-tricarboxylic acid(PBTC), hydroxyphosphonoacetic acid, amino phosphonates; terpolymers,copolymers and homopolymers of acrylic acids, maleic acids; and blendsthereof.

SUMMARY OF THE INVENTION

The present invention provides an effective method for enhancing theinhibitory effect of individual or blends of known scale formation,corrosion, and deposition control agents in aqueous systems by employinga combination of known scale control materials and one or moresubstituted alkyl polycarboxylates. The combination of the presentinvention provides for enhanced scale/corrosion inhibiting and depositcontrol properties in aqueous systems. The combinations of the presentinvention provide for scale/corrosion formation inhibition and depositcontrol in circulating aqueous systems which are effective at conditionsof high pH, high scale forming ion concentration and high M-alkalinitywhere conventional control treatments often lose efficacy. The presentinvention controls scale/corrosion formation and the fouling of heattransfer surfaces. The present invention is effective at treating watershaving low levels of scale forming ions as well as those having highscale forming ion levels.

The present invention is effective at inhibiting the formation anddeposition of scale forming materials such as calcium oxalate, calciumsulfate, barium sulfate as well as the more common calcium carbonate.The present invention is effective at controlling scale deposition onmetal surfaces such as Admiralty brass, copper, stainless steel and mildsteel, as well as plastics such as polyvinyl chloride (PVC) andpoly(methylmethacrylate). The present invention is effective atinhibiting calcium carbonate at high pH as would be required in papermills. The method of the present invention comprises treating industrialwater with a combination of one or more scale/corrosion inhibitor suchas phosphonates (hydroxyethylidene diphosphonic acid,2-phosphonobutane-1,2,4-tricarboxylic acid, hydroxyphosphonoacetic acid,amino phosphonates, etc. ), terpolymers, copolymers or homopolymers ofacrylic acids, maleic acids, or an epoxysuccinic acids such aspolyepoxysuccinic acid, of the general formula: ##STR2## wherein nranges from about 2 to 50, preferably 2 to 25; M is hydrogen or a watersoluble cation such as Na⁺, NH₄ ⁺ or K⁺ and R is hydrogen, C1-C4 alkylor C1-C4 substituted alkyl (preferably R is hydrogen), and a substitutedalkylpolycarboxylate.

In the present invention, the combination is added to aqueous systems atsubstoichiometric levels to inhibit scale formation and depositionand/or corrosion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a novel method of inhibiting theformation and deposition of scale such as calcium scale from circulatingaqueous systems. Specifically, the present invention comprises adding toa circulating aqueous system a treatment solution comprising acombination of a scale/corrosion inhibitor such as phosphonates(hydroxyethylidene diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, hydroxyphosphonoacetic acid, aminophosphonates, etc. ), terpolymers, copolymers or homopolymers of acrylicacids, maleic acids, or an epoxysuccinic acid such as polyepoxysuccinicacids, of the general formula: ##STR3## wherein n ranges from about 2 to50, preferably 2 to 25; M is hydrogen or a water soluble cation such asNa⁺, NH₄ ⁺ or K⁺ and R is hydrogen, C1-C4 alkyl or C1-C4 substitutedalkyl (preferably R is hydrogen), and one or more substitutedalkylpolycarboxylates.

Polyepoxysuccinic acids are known to provide scale inhibition comparableto prior art phosphates, phosphonates and polyacrylates without therecognized limitations of the prior art treatments. Polyepoxysuccinicacids are effective in most water systems, and particularly effective inaqueous systems having high Langelier Saturation Index (LSI) numbers.U.S. Pat. No. 5,062,962 (incorporated herein by reference) outlines amethod of preparing a polyepoxysuccinic acid material useful in thepresent invention. It will be appreciated that by-products, for exampledisodium tartrate, can be co-produced along with the polyepoxysuccinicacid in the course of the referenced reaction scheme. Thepolyepoxysuccinic acid can be readily recovered from the reactionproduct by known methods; however, it is feasible and economical toemploy the material as produced without separation or purification.

The substituted alkylpolycarboxylates of the combination of the presentinvention are compounds of the general structure:

    R--X--Y.sub.n

wherein R is a hydrocarbon functionality having 4 to 18 carbons; X, whenpresent, represents --NH--, --NY¹ --, --N-->0,--N--OC--Y¹, --O--, or--S--; n is greater than or equal to 1; Y and Y¹ each independentlyrepresent a substituted or non-substituted carboxylic acid functionalitysuch that said compound contains a total of at least 2 carboxylic acidfunctional groups. Table I sets forth exemplary substitutedalkylpolycarboxylates of the present invention.

                                      TABLE I                                     __________________________________________________________________________    Compound    R          X     Y               Y.sup.1                          __________________________________________________________________________    AA(N-DDC)   CH.sub.3(CH.sub.2).sub.11                                                                S                                                                                    ##STR4##       --                               C.sub.n -IDA                                                                              C.sub.4 H.sub.9                                                                          N     (CH.sub.2CO.sub.2 Na).sub.2                                                                   --                                           C.sub.8 H.sub.17                                                                         N     (CH.sub.2CO.sub.2 Na).sub.2                                                                   --                                           C.sub.12 H.sub.25                                                                        N     (CH.sub.2CO.sub.2 Na).sub.2                                                                   --                               C.sub.12 -IDA oxide                                                                       C.sub.12 H.sub.25                                                                         ##STR5##                                                                           (CH.sub.2 CO.sub.2 H).sub.2                                                                   --                               C.sub.8O-Citric.3Na                                                                       CH.sub.3 (CH.sub.2).sub.7                                                                O                                                                                    ##STR6##       --                               TBCMAPG     tallow                                                                                    ##STR7##                                                                           [CH.sub.2 CO.sub.2 H]                                                                         [(CH.sub.2).sub.3N(CH.sub.2CO                                                 .sub.2 H).sub.2 ]                OBCMAPG     Oleyl                                                                                     ##STR8##                                                                           [(CH.sub.2CO.sub.2 H).sub.2 ]                                                                 [(CH.sub.2).sub.3N(CH.sub.2                                                   CO.sub.2 H).sub.2 ]              Duomeen T-HSA                                                                             tallow                                                                                    ##STR9##                                                                            ##STR10##                                       Deriphat 160-C                                                                            C.sub.12 H.sub.25                                                                        N     (CH.sub.2CH.sub.2CO.sub.2 H/Na).sub.2                                                         --                               Deriphat 154-L                                                                            tallow     N     (CH.sub.2CH.sub.2CO.sub.2 H/Na).sub.2                                                         --                               TBCEAPA.Na.sub.3                                                                          tallow                                                                                    ##STR11##                                                                          [CH.sub.2 CH.sub.2 CO.sub.2 Na]                                                               [(CH.sub.2).sub.3 N((CH.sub.2                                                 ).sub.2 CO.sub.2 Na).sub.2                                                    ]                                C.sub.4 ASP.2Na                                                                           C.sub.4 H.sub.9                                                                          NH                                                                                   ##STR12##      --                               C.sub.8 ASP.2Na                                                                           C.sub.8 H.sub.17                                                                         NH                                                                                   ##STR13##      --                               C.sub.12 ASP.2Na                                                                          C.sub.12 H.sub.25                                                                        NH                                                                                   ##STR14##      --                               C.sub.18 ASP.2Na                                                                          C.sub.18 H.sub.37                                                                        NH                                                                                   ##STR15##      --                               C.sub.12 NHCH.sub.2 -Succ.2Na                                                             CH.sub.3 (CH.sub.2).sub.11                                                               NH                                                                                   ##STR16##      --                               Monawet SNO35 (Mona Industries)                                                           C.sub.18 (alkyl)                                                                          ##STR17##                                                                           ##STR18##                                                                                     ##STR19##                       Agaric Acid CH.sub.3(CH.sub.2).sub.15                                                                --                                                                                   ##STR20##      --                               __________________________________________________________________________

In the testing of the scale control properties of inhibitors, the testmethod measures surface deposition or surface fouling in a dynamic orcirculating system. The inventors of the present invention discoveredthat the efficacy of scale/corrosion inhibitors at controlling surfacedeposition and bulk phase precipitation in aqueous systems was enhancedby the co-addition of substituted alkylpolycarboxylates.

The treatment levels of scale/corrosion inhibitors added to an aqueoussystem can range from 25 parts per billion to about 500 parts permillion. The preferred treatment levels range from about 1 part permillion up to about 100 parts per million. The concentration ofscale/corrosion inhibitor necessary to provide effective scale controlwill vary from system to system. The treatment will vary, in part, withchanges in temperature, pH and scale forming ion concentration. However,in all cases, the concentration of scale/corrosion inhibitor added to acirculating aqueous system in accordance with the present invention is asubstoichiometric concentrations. That is, the concentration ofscale/corrosion inhibitor added is much lower than the concentration ofthe scale forming material in the system to be treated.

The treatment levels of substituted alkylpolycarboxylate added to thesystem can range from about 20 parts per billion up to about 50 partsper million. The preferred treatment level ranges from about 0.1 partsper million up to about 10 parts per million. The ratio ofscale/corrosion inhibitor to substituted alkylpolycarboxylate can rangefrom about 100 to 1 to about 1 to 10.

The present invention will now be described with reference to a numberof specific examples which are to be regarded solely as illustrative andnot as restricting the scope of the present invention.

EXAMPLES Example 1

Dynamic beaker calcium carbonate inhibition testing at varying treatmentlevels were undertaken. The testing involved adding the treatment to asolution containing calcium and carbonate ions and having a pH of 8.7 at56° C. Coupons of the materials to be tested were suspended in thesolution. The beakers were incubated in a shaking water bath for 66hours at 56° C. After cooling, a measured portion was filtered, and thepH adjusted to less than 2.0 with hydrochloric acid. The mixture wasdiluted and the pH adjusted to 12 with sodium hydroxide. A calciumindicator, Murexide, was added and the solution titrated to a pureviolet endpoint with ethylenediamine tetraacetic acid. Percentinhibition was calculated from titrations of the treated, stock andcontrol solutions. Deposition on the suspended surfaces was evaluated byvisual inspection. The conditions of the tests were: 600 ppm Ca as CaCO₃; 300 ppm Mg as CaCO₃ ; 400 ppm Malk as CaCO₃ ; 288 ppm SO₄ ; 425 ppmCl; 186 ppm Ma; pH 8.7 at 56° C.; 3 ppm tolyltriazole (added to preventyellow metal corrosion).

Table II summarizes the results of the tests and shows that thecombinations of the present invention improve the deposit rating for allof the materials tested.

                  TABLE II                                                        ______________________________________                                                                             Depo-                                    ppm               Percent  Coupon    sition*                                  active - Treatment                                                                              Inhibition                                                                             Material  Rating                                   ______________________________________                                        Control           --       Admiralty 4.0                                      Control           --       Plexiglas 4.0                                      10-PESA           54.0     Admiralty 2.0                                      20-PESA           83.0     Admiralty 2.0                                      10-PESA           63.3     Plexiglas 3.0                                      20-PESA           84.0     Plexiglas 4.0                                      10-PESA + 2-Monawet sno-35                                                                      93.0     Plexiglas 1.5                                      20-PESA + 3-Monawet sno-35                                                                      95.4     Plexiglas 1.0                                      20-PESA + 4-Monawet sno-35                                                                      89.4     Plexiglas 1.0                                      20-PESA + 10-Monawet sno-35                                                                     90.9     Plexiglas 1.0                                      10-PESA + 2-Monawet sno-35                                                                      87.8     Admiralty 1.0                                      20-PESA + 2-Monawet sno-35                                                                      87.0     Admiralty 1.0                                      20-PESA + 4-Monawet sno-35                                                                      80.8     Admiralty 1.0                                      20-PESA + 10-Monawet sno-35                                                                     87.8     Admiralty 1.0                                      6-HEDP + 10-HPS I**                                                                             84.4     Admiralty 2.0                                      6-HEDP + 10-HPS I +                                                                             84.0     Admiralty 1.0                                      2-Monawet sno-35                                                              6-HEDP + 10 HPS I 93.0     Plexiglas 1.5                                      6-HEDP + 10 HPS I +                                                                             91.0     Plexiglas 1.0                                      2-Monawet sno-35                                                              10-PESA + 2-AA(N-DDC)                                                                           77.3     Plexiglas 3.0                                      20-PESA + 2-AA(N-DDC)                                                                           82.4     Plexiglas 3.0                                      10-PESA + 2-C4-IDA                                                                              73.1     Plexiglas 3.0                                      20-PESA + 2-C4-IDA                                                                              71.0     Plexiglas 3.0                                      10-PESA + 2-C4-IDA                                                                              78.4     Admiralty 1.0                                      20-PESA + 2-C4-IDA                                                                              90.4     Admiralty 1.0                                      10-PESA + 2-C8-IDA                                                                              74.9     Plexiglas 3.0                                      20-PESA + 2-C8-IDA                                                                              80.5     Plexiglas 2.5                                      10-PESA + 2-C8-IDA                                                                              71.9     Admiralty 1.0                                      20-PESA + 2-C8-IDA                                                                              89.1     Admiralty 1.0                                      10-PESA + 2-C12-IDA                                                                             82.5     Plexiglas 3.0                                      20-PESA + 2-C12-IDA                                                                             83.9     Plexiglas 2.0                                      10-PESA + 2-C12-IDA                                                                             77.4     Admiralty 1.0                                      20-PESA + 2-C12IDA                                                                              90.4     Admiralty 1.0                                      10-PESA +         90.4     Plexiglas 2.0                                      2-C8--O-CITRIC.3NA                                                            10-PESA +         96.8     Plexiglas 1.0                                      4-C8--O-CITRIC.3NA                                                            20-PESA +         92.2     Plexiglas 1.0-3.0                                  2-C8--O-CITRIC.3NA                                                            20-PESA +         92.3     Plexiglas 1.0                                      4-C8--O-CITRIC.3NA                                                            10-PESA +         66.1     Admiralty 1.0-2.0                                  2-C8--O-CITRIC.3NA                                                            20-PESA +         90.6     Admiralty 1.0                                      2-C8--O-CITRIC.3NA                                                            10-PESA + 2-TBCMAPG                                                                             100.0    Plexiglas 1.0                                      20-PESA + 2-TBCMAPG                                                                             99.5     Plexiglas 1.0                                      10-PESA + 2-TBCMAPG                                                                             70.7     Admiralty 1.0                                      20-PESA + 2-TBCMAPG                                                                             86.3     Admiralty 1.0                                      10-PESA + 2-OBCMAPG                                                                             100.0    Plexiglas 1.0                                      20-PESA + 2-OBCMAPG                                                                             98.5     Plexiglas 1.0                                      10-PESA + 2-OBCMAPG                                                                             55.6     Admiralty 3.0                                      20-PESA + 2-OBCMAPG                                                                             65.2     Admiralty 3.0                                      10-PESA + 2-Duomeen T-HSA                                                                       89.0     Plexiglas 1.0                                      20-PESA + 2-Duomeen T-HSA                                                                       92.6     Plexiglas 1.0                                      10-PESA + 2-Duomeen T-HSA                                                                       71.9     Admiralty 2.0                                      20-PESA + 2-Duomeen T-HSA                                                                       77.9     Admiralty 1.0                                      10-PESA + 2-Deriphat 160-C                                                                      74.9     Plexiglas 3.0                                      20-PESA + 2-Deriphat 160-C                                                                      78.0     Plexiglas 2.0                                      10-PESA + 2-Deriphat 160-C                                                                      57.9     Admiralty 1.0                                      20-PESA + 2-Deriphat 160-C                                                                      76.4     Admiralty 2.0                                      10-PESA + 2-Deriphat 154-L                                                                      93.0     Plexiglas 1.0                                      20-PESA + 2-Deriphat 154-L                                                                      87.0     Plexiglas 1.0                                      10-PESA + 2-Deriphat 154-L                                                                      79.8     Admiralty 1.0                                      20-PESA + 2-Deriphat 154-L                                                                      72.0     Admiralty 1.0                                      10-PESA + 2-TBCEAPA.NA3                                                                         93.7     Plexiglas 1.0                                      20-PESA + 2-TBCEAPA.NA3                                                                         87.3     Plexiglas 1.0                                      10-PESA + 2-TBCEAPA.NA3                                                                         88.7     Admiralty 2.0                                      20-PESA + 2-TBCEAPA.NA3                                                                         87.0     Admiralty 2.0                                      20-PESA + 2-C4-Asp.2Na                                                                          76.6     Plexiglas 4.0                                      20-PESA + 2-C4-Asp.2Na                                                                          74.3     Admiralty 2.0                                      20-PESA + 2-C8-Asp.2Na                                                                          73.0     Plexiglas 4.0                                      20-PESA + 2-C8-Asp.2Na                                                                          82.7     Admiralty 2.0                                      20-PESA + 2-C12-Asp.2Na                                                                         78.6     Plexiglas 3.0-4.0                                  20-PESA + 2-C12-Asp.2Na                                                                         84.0     Admiralty 1.0                                      20-PESA + 2-C18-Asp.2Na                                                                         84.1     Plexiglas 3.0                                      20-PESA + 2-C18-Asp.2Na                                                                         4.5      Admiralty 1.0                                      10-PESA +         81.6     Plexiglas 3.0                                      2-C12--NH--CH2-SUCC.2NA                                                       20-PESA +         80.6     Plexiglas 2.0-3.0                                  2-C12--NH--CH2-SUCC.2NA                                                       10-PESA +         58.5     Admiralty 3.0                                      2-C12--NH--CH2-SUCC.2NA                                                       20-PESA +         77.8     Admiralty 2.0                                      2-C12--NH--CH2-SUCC.2NA                                                       10-PESA + 2-Agaric Acid                                                                         66.5     Plexiglas 2.0                                      20-PESA + 2-Agaric Acid                                                                         80.0     Plexiglas 1.0                                      10-PESA +  2-Agaric Acid                                                                        65.0     Admiralty 2.0                                      20-PESA + 2-Agaric Acid                                                                         80.0     Admiralty 1.0                                      10-PESA + 2-C12 IDA oxide                                                                       82.6     Plexiglas 4.0                                      20-PESA + 2-C12 IDA oxide                                                                       87.1     Plexiglas 3.0                                      2-Monawet sno-35  6.9      Admiralty 4.0                                      5-Monawet sno-35  -7.5     Admiralty 4.0                                      10-Monawet sno-35 -5.0     Admiralty 4.0                                      20-Monawet sno-35 -0.9     Admiralty 4.0                                      20-C8--O-CITRIC.3NA                                                                             -4.7     Plexiglas 3.0                                      20-C8--O-CITRIC.3NA                                                                             8.2      Admiralty 4.0                                      10-C8-IDA         -10.3    Admiralty 4.0                                      10-C8-IDA         -9.6     Admiralty 4.0                                      10-C12-IDA        -16.9    Admiralty 4.0                                      10-C12-IDA        -17.1    Admiralty 3.0-4.0                                  20-TBCEAPA.NA3    8.6      Plexiglas 2.0                                      20-TBCEAPA.NA3    7.5      Admiralty 4.0                                      2-Agaric Acid     3.0      Admiralty 3.0                                      ______________________________________                                         *1 = Clean; 2 = Very Slight Deposit; 3 = Slight Deposit; 4 = Moderate         Deposit                                                                       **HPS I is a copolymer of acrylic acid and allylhydroxypropylsulfonate        ether sodium salt available from Betz Laboratories, Inc.                 

Example 2

Testing was conducted in a bench top unit (BTU) which simulates acirculating cooling water system. The BTU units are designed to providea measure of the ability of the treatment to prevent corrosion and scaleformation under heat transfer conditions. The treated water iscirculated through a corrosion coupon by-pass rack, into which corrosioncoupons are inserted and passes through a heat exchanger tube containedin a Plexiglas® block (trademark of Rohm & Haas Company). The heatexchanger is fitted with an electrical heater so that the heat load onthe exchanger tube can be varied and controlled in the 0 to 16,000BTU/ft² /hr range. The water velocity passing the unit can be controlledin the 0 to 4.5 ft/sec.range. A section of PVC tower fill is immersed inthe sump of the unit.

The pH and temperature of the circulating water are automaticallycontrolled. The treated water was prepared by the addition of thecomponent ions to deionized water. Provisions for continuous makeup andblowdown are made by pumping fresh treated water from supply tanks tothe sump of the unit, with overflow from the sump serving as blowdown.The system volume is approximately 12 liters.

Table III summarizes testing in these bench top units for a variety ofmaterials. The test conditions were: 600 ppm Ca as CaCO₃ ; 300 ppm Mg asCaCO₃ ; 400 ppm Malk as CaCO₃ ; 288 ppm SO₄ ; 425 ppm Cl; 187 ppm Na; 3ppm tolyltriazole; pH 8.8; bulk temperature 49° C.; skin temperature 57°C.; flow rate 2.8 feet/second; heat input 13,000 BTU/ft² /hr. The heattransfer surfaces and coupons were admiralty brass.

                                      TABLE III                                   __________________________________________________________________________                   Tube*                                                                              Plexiglas                                                                            PVC Tower*                                                                            Coup.*                                               ppm  Depos.                                                                             Block Dep.                                                                           Fill Dep.                                                                             Dep. Duration                                                                            Turbidity                       Treatment Active                                                                             Rating                                                                             Rating Rating  Rating                                                                             Days  (NTU)                           __________________________________________________________________________    PESA      20   4.0  4.0    4.0     4.0  7     5.86                            PESA      20   1.5N 1.0    1.0     1.5  7     0.81                            Monawet sno-35                                                                          2                                                                   PESA      20   1.0  1.0    1.0     1.0  7     0.50                            Agaric Acid                                                                             1                                                                   HEDP      6    2.5U N/A    2.0U    2.0U 7     --                              HPS II**  10                                                                  HEDP      6    1.0  N/A    1.0     1.0  7     1.74                            HPS II**  10                                                                  Monawet sno-35                                                                          2                                                                   Acumer 4200                                                                             20   3.0N N/A    1.0     1.0  7     1.14                            (Rohm & Haas)                                                                 Acumer 4200                                                                             20   1.5N N/A    1.0     1.0  7     1.14                            Monawet sno-35                                                                          2                                                                   __________________________________________________________________________     *1 = Clean; 2 = Very Slight Deposit; 3 = Slight Deposit; 4 = Moderate         Deposit                                                                       N = Nonuniform Deposit                                                        U = Uniform Deposit                                                           N/A = A PVC Deposition Block was used instead of Plexiglas                    **HPS II is a copolymer of acrylic acid and allylhydroxypropylsulfonate       ether sodium salt available from Betz Laboratories, Inc.                 

Table IV summarizes testing in these bench top units and shows thecorrosion and deposit control activity of the present invention. Thetest conditions were: 600 ppm Ca as CaCO₃ ; 300 ppm Mg as CaCO₃ ; 400ppm Malk as CaCO₃ ; 288 ppm SO₄ ; 425 ppm Cl; 187 ppm Na; 3 ppmtolyltriazole; pH 8.8; bulk temperature 49° C.; skin temperature 57° C.;fbw rate 2.8 feet/second; heat input 13,000 BTU/ft² /hr. The heattransfer surfaces and coupons were low carbon steel.

                                      TABLE IV                                    __________________________________________________________________________                   Tube*                                                                              Plexiglas                                                                            PVC Tower*                                                                            Coup.*                                               ppm  Depos.                                                                             Block Dep.                                                                           Fill Dep.                                                                             Dep.  Duration                                                                            Turbidity                      Treatment Active                                                                             Rating                                                                             Rating Rating  Rating                                                                              Days  (NTU)                          __________________________________________________________________________    PESA      20   2.0U 5.0    3       3     7     1.2                                                               (2.4 mpy)                                  PESA      20   1.5U 1      1       1.5U  7     1.3                            Agaric Acid                                                                             1                        (1.2 mpy)                                  PESA      20   1.5N 1.5    1       2.0U  7     0.5                            Monawet sno-35                                                                          2                        (1.3 mpy)                                  __________________________________________________________________________     *1 = Clean; 2 = Very Slight Deposit; 3 = Slight Deposit; 4 = Moderate         Deposit                                                                       N = Nonuniform Deposit                                                        U = Uniform Deposit                                                           N/A = A PVC Deposition Block was used instead of Plexiglas               

Table V summarizes the results of the bench top unit testing and showsthat the combinations of the present invention enhance the corrosion anddeposit control activity in circulating systems. The test conditionswere: 600 ppm Ca as CaCO₃ ; 300 ppm Mg as CaCO₃ ; 500 ppm Malk as CaCO₃; 288 ppm SO₄ ; 425 ppm Cl⁻ ; 231 ppm Na; 3 ppm tolyltriazole; pH 9.0;bulk temperature 49° C.; skin temperature 57° C.; flow rate 2.8feet/second; heat input 13,000 BTU/ft² /hr. The heat transfer surfacesand coupons were admiralty brass.

                  TABLE V                                                         ______________________________________                                                               PVC                                                                   Tube*   Tower* Coup.*                                                                              Dura-                                     Treat- ppm     Depos.  Fill Dep.                                                                            Dep.  tion  Turbidity                           ment   Active  Rating  Rating Rating                                                                              Days  (NTU)                               ______________________________________                                        PESA   6.0     5N      4N     4N     8    59.9                                PBTC   6.0                                                                    HPS II 20.0                                                                   PESA   6.0     1       1      1     16     2.7                                PBTC   6.0                                                                    HPS II 15.0                                                                   Monawet                                                                              2.0                                                                    sno-35                                                                        ______________________________________                                         *1 = Clean; 4 = Moderate Deposit; 5 = Heavy Deposit                           N = Nonuniform Deposit                                                   

While the present invention has been described with respect toparticular embodiments thereof, it is apparent that numerous other formsand modifications of the invention will be obvious to those skilled inthe art. The appended claims and this invention generally should beconstrued to cover all such obvious forms and modifications which arewithin the true scope and spirit of the present invention.

I claim:
 1. A method of controlling corrosion and the formation anddeposition of calcium scale forming salts in an aqueous systemcomprising introducing into said aqueous system a substoichiometricamount sufficient for the purpose of a treatment solution comprising (a)a scale/corrosion inhibitor selected from the group consisting ofphosphonates, terpolymers, copolymers and homopolymers of acrylic acids,maleic acids and epoxysuccinic acid based polymers or mixtures thereofand (b) a substituted alkylpolycarboxylate of the general structureR--X--Y_(n) wherein R is a hydrocarbon functionality having 4 to 18carbons; X, is --NH--, --NY¹ --, --N-->O, --N--OC--Y¹, --O-- or --S; nis greater than or equal to 1; and Y and Y¹ each independently representOH, SO₃ Na, (CH₂)₃ --N-- substituted or non-substituted carboxylic acidfunctionality such that said alkylpolycarboxylate has at least 2carboxylic acid functional groups wherein the ratio of (a):(b) is fromabout 100:1 to about 1:10.
 2. The method of claim 1 wherein saidphosphonate is selected from the group consisting ofhydroxyethylidenediphosphonic acid,2-phosphonobutane-1,2,4-tricarboxylic acid, hydroxyphosphonoaceticacids, and aminophosphonates.
 3. The method of claim 1 wherein saidscale/corrosion inhibitor is added to the aqueous system at activetreatment levels ranging from about 25 parts per billion to about 100parts per million and said substituted alkylpolycarboxylate is added tothe aqueous system at active treatment levels ranging from about 20parts per billion up to about 50 parts per million.
 4. The method ofclaim 1 wherein said scale/corrosion inhibitor is selected from thegroup consisting of an acrylic acid/allylhydroxypropylsulfonatecopolymer; or a polyepoxysuccinic acid of the formula: ##STR21## whereinn is from about 2 to 50 and M is a water soluble cation or mixturesthereof.